Charged particle beam apparatus including aberration corrector

ABSTRACT

A focused charged particle beam apparatus including an aberration corrector, capable of finding the absolute value of the aberration coefficient at high speed, and capable of making high-accuracy adjustments at high speed. A deflection coil tilts the input beam relative to the object point, and measures the defocus data and aberration quantity at high speed while the beam is tilted from one image, and perform least squares fitting on these results to find the absolute value of the aberration coefficient prior to tilting the beam, and to adjust the aberration corrector.

CLAIM OF PRIORITY

The present application claims priority from Japanese patent applicationJP 2008-040815 filed on Feb. 22, 2008, the content of which is herebyincorporated by reference into this application.

FIELD OF THE INVENTION

The present invention relates to a charged particle beam apparatusincluding an aberration corrector, and relates in particular to anautomatic adjustment method for scanning electron microscopes (SEM) andscanning transmission electron microscopes (STEM). This invention alsorelates in particular to automatic adjustment of scanning chargedparticle beam apparatuses containing high-order aberration correctorscapable of compensating for so-called high-order aberrations such asthird-order and fifth-order aberrations.

BACKGROUND OF THE INVENTION

Devices using focused charged particle beams (probe beams) such asscanning electron microscopes (SEM) and focused ion beam (FIB) irradiatea probe beam onto the specimen to make image observations and machinethe specimen. Here, the size of the probe cross-section (probe diameter)determines the resolution and machining precision of these chargedparticle beam apparatuses and basically, the smaller the probe crosssection, the higher the resolution and machining precision. Progress hasbeen made in recent years toward developing aberration correctors forfocused charged particle beam apparatuses that are now reaching thepractical application stage. The aberration corrector utilizes amulti-electrode lens to apply a magnetic field and a non-rotationallysymmetrical magnetic field to the beam to give the probe beam an inverseaberration. The aberration corrector in this way cancels out differenttypes of aberrations (or aberrations) such as chromatic aberrations orspherical aberrations generated by the objective lens or deflector lensin the optical system.

Optical systems in devices such as focused charged particle beams of therelated art use a rotationally symmetric axial lens. Essentially, theprobe diameter can be adjusted to a super-small value by aligning eachlens axis and axial aperture, and adjusting the focus and aberration. Toadjust the focus and correct the aberration, the probe image wasacquired under the condition that the focus was changed and adjustmentmade by selecting the location with the highest degree of sharpnesswhile comparing the degree of sharpness in the image in at least twodimensions. On the other hand, in devices using focused charged particlebeams including aberration correctors, a magnetic field andnon-rotationally symmetrical magnetic field were applied by anaberration corrector using a multi-electrode lens. So even thoughhigher-order aberrations do not affect rotationally symmetrical opticalsystems in the related art, these higher-order aberrations do exert adrastic effect on focused charged particle beam apparatuses. Extractingthe maximum possible level of device performance requires accuratelymeasuring the type (aberration component) of aberrations in the beam aswell as these higher-order aberrations and the quantity of eachaberration component and then removing all aberration components byadjusting the aberration corrector as needed.

Directly observing the cross sectional shape of the probe in focusedcharged particle beam apparatuses such as SEM and FIB that use probebeams is impossible. A method of the known art therefore extractsinformation on the probe shape by processing the image in a state wherethe image from the specimen contains the aberration. The type andquantity of aberration is then found by identifying discrepancies forexample in the size, contour, and luminance of a probe shape thatcontains no aberrations.

In a method for extracting the probe shape as disclosed inJP-T-2003-521801 laid open (patent document 1), specimen structuralinformation is deleted by dividing out the specimen image whileunderfocused (state where beam converges rearward of specimen) oroverfocused (state where beam converges forward of specimen) in theFourier space from the specimen image while exactly focused (beam isfocused on specimen material) to in this way obtain the probe shape. Inthis method, the probe is gradually made visual while amplifying theaberration component information contained in the probe by shifting thefocus. The method in JP-A-2005-183086 laid open (patent document 2)discloses in detail a method for setting the aberration type andquantity from the shape of the probe obtained by the above describedtechnique. The probe shape is in this way found in the underfocus andoverfocus states, multiple lines are drawn at equiangular gaps from themedian point of the probe shape just obtained, and the line profileinformation is extracted. Unique quantities expressing the line profilewidth, bilateral asymmetry, and irregularities (concavities/protrusions)near the center are then found. These unique quantities are next changedby way of the line angles and focus states such as under-focus andover-focus when aberrations are present, and defined as parametersexpressing geometrical aberrations and parasitic aberrations (defocus,first-to-third order aberrations, coma, spherical surfaces, frames,stars) up to third-order aberrations where the unique quantities are setas variables for and these quantities are utilized as guides forexpressing different aberrations.

In transmission electron microscopes (TEM) on the other hand, an imagecalled a diffractogram is obtained by making Fourier transforms of theamorphous specimen image using an electron beam whose input angle wastilted away from the objective lens optical axis. The diffractogramshape reveals effects from the aberration and so has long been used inattempts to find the aberration coefficient by utilizing thediffractogram image interaction.

A method for example in Ultramicroscopy 81 (2000), pp. 149-161 isdisclosed that finds the aberration coefficient by measuring the size ofthe defocus and aberration from the sloping beam diffractogram andsolving the inverse problem. A method is disclosed in Optik 99 (1995)pp. 155-166 for finding the aberration coefficient by calculating theamount of image movement via the beam tilt by calculating theinteraction of two images captured under different beam tilt conditions,and solving the inverse problem.

SUMMARY OF THE INVENTION

The values obtained from methods disclosed in JP-T-2003-521801 andJP-T-2005-183086 for calculating the aberration from underfocus andoverfocus probe shapes, express quantities that serve as a guide forshowing the relative size of aberrations and are not the aberrationcoefficients that are defined for wave optics and geometrical objects.So using values from these methods as logical values for an aberrationor as comparisons for verifying documents or other devices isimpossible. These values are also not usable for making absoluteevaluations of accuracy so there is a limit to how far that the accuracyof the compensator/corrector can be adjusted. Moreover, these methodsalso have the problem that another calculation technique must be used tofind the aberration coefficient value after correcting the aberration.

Unlike transmission electron microscopes (TEM), scanning chargedparticle beam apparatuses cannot acquire diffractograms and so mustidentify the aberration information from just the specimen image.Scanning charged particle beam apparatuses operate by focusing andirradiating a charged electron beam onto the specimen, and aberrationinformation that the image contains is also focused. So scanning chargedparticle beam apparatuses, unlike diffractograms cannot determine thesize of the aberration from one image. Techniques relying on a tiltedbeam such as utilized in conventional TEM are therefore difficult toapply to scanning charged particle beam apparatuses.

In view of the above problems with the related art, this invention hasthe object of providing a focused charged particle beam apparatuscapable of finding the aberration coefficient absolute value, and usingthis aberration coefficient to make high precision aberration correctoradjustments.

This invention scans a charged particle beam tilted along themeasurement specimen or the scanning surface of the specimen andacquires the secondary charged particle signal, sets the defocus dataand aberration quantity of the tilt from the secondary intensitydistribution information of the secondary charged particle signal thatwas obtained and, finds the aberration coefficient from the obtaineddefocus data and aberration quantities. The aberration coefficientobtained here is then utilized to adjust the aberration corrector.

The method for calculating the aberration coefficient as described aboveutilizes two basic principles. A first principle is that aberrationsincrease the change in the optical path differential due to the beamtilt. A second principle is that the extent to which the aberrationchanges due to the beam tilt is dependent on the extent of originalaberration present in the beam tilt conditions and the beam irradiationsystem. In other words, the technique for calculating the aberrationcoefficient of this invention is characterized in measuring the defocusdata and aberration quantity tilt condition dependence and, inverselycalculating the aberration coefficient from the measured dependence in anon-tilt state. The “tilt conditions” for tilt condition dependence arehere expressed by the azimuth and tilt angle of the charged particlebeam relative to the specified optical axis (i.e. the center axis of thecharged particle column and the optical axis of the objective lens,etc.).

This invention can therefore adjust the aberration corrector at highspeed by obtaining the absolute value for aberration coefficients in theoptical systems of charged particle beam apparatuses.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a graph showing the least squares fitting and the C1(τ)measurement value for finding the aberration coefficient;

FIG. 2 is a block diagram showing the device of the first embodiment;

FIG. 3 is a graph drawing showing the step-shaped standard specimen;

FIG. 4 is a flow chart of the overall operation of the device of thefirst embodiment;

FIG. 5 is a drawing showing an example of the specimen image when thefocus was aligned using the stair-shaped specimen stand;

FIG. 6 is a detailed flow chart showing the aberration coefficientmeasurement flow for the flow chart shown in FIG. 4;

FIG. 7 is a drawing showing the change in the specimen image when thebeam was tilted, and the interrelation between the defocus data and theaberration quantity;

FIG. 8 is a block diagram showing the structure of the device for thesecond embodiment;

FIG. 9 is a block diagram showing the structure of the device for thethird embodiment; and

FIG. 10 is a drawing showing the interface screen for the device for thethird embodiment.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Principles jointly used in the embodiments as techniques for calculatingthe aberration coefficient are described next before proceeding with thedescription of the individual embodiments.

In the technique described in the embodiments for measuring theaberration coefficient, the axis of the input charged particle beamscans the specimen while tilted from the Z-axis and acquirestwo-dimensional pixel information on the secondary charged particles.The pixel two-dimensional distribution information synchronized with thedeflector frequency of the scanning deflector device and arrayed aspixels are the so-called “image”. This operation to acquiretwo-dimensional pixel information and change the tilt azimuth of theinput beam is executed multiple times, and two-dimensional distributioninformation scanned along a different azimuth angle by the primaryparticle beam is acquired multiple times. Acquiring two-dimensionaldistribution information by using a tilt beam is physically equivalentto enhancing the aberrations originally contained in the probe beam. Thebeam input along the optical axis is in a state where all aberrationsoverlap on the optical system, and extracting a specified aberrationquantity at this point is impossible. However, tilting this beam makesthe different aberration components appear at distinctly differentfrequencies according to the tilt azimuth. The multiple aberrationcomponents can therefore be easily identified and their aberrationquantities found after tracing changes in the image by capturing imagesof the sloping beam at different azimuth angles.

The defocus data and astigmatic difference contained in each of thetwo-dimensional distribution information obtained above are next found.The “defocus data” described here signifies the difference in focuspositions between the in-focus state and the defocus state. The“astigmatic difference” signifies the difference in focal pointdistances in two intersecting directions along the optical axis of thecharged particle beam.

Measuring the defocus data and astigmatic quantity at each azimuthangle, while changing the azimuth bearing, yields a data string forthese defocus data items and astigmatic quantities. An absolute valuefor the aberration coefficient can be calculated by substituting thisdata string into a formula expressing the defocus and aberration duringthe beam tilt, and then performing least squares fitting.

Irradiating the beam onto the object point in a state where possessing afixed tilt angle, generates an optical path discrepancy in the electronbeam due to the beam tilt, and this beam tilt adds the aberration to thespecimen image. Setting a function to express this optical pathdiscrepancy due to the aberration as χ(ω), usually allows χ(ω) toanalytically express the discrepancy by using multiple order aberrationcoefficients. Here, writing χ(ω) up to a third order aberrationcoefficient is expressed in the following formula (Formula 1).

$\begin{matrix}{{\chi (\omega)} = {{Re}\begin{Bmatrix}{{\frac{1}{2}\omega \; \overset{\_}{\omega}\; C_{1}} + {\frac{1}{2}{\overset{\_}{\omega}}^{2}A_{1}} + {\omega^{2}\overset{\_}{\omega}\; B_{2}} + {\frac{1}{3}{\overset{\_}{\omega}}^{3}A_{2}} +} \\{{{\frac{1}{4}\omega^{2}{\overset{\_}{\omega}}^{2}C_{3}} + {\omega^{3}\overset{\_}{\omega}\; S_{3}} + {\frac{1}{4}{\overset{\_}{\omega}}^{4}A_{3}} + \ldots}\mspace{14mu}}\end{Bmatrix}}} & {{Formula}\mspace{14mu} 1}\end{matrix}$

In Formula 1, C1, A1, B2, A2, C3, S3, A3 respectively denote thedefocus, two-fold astigmatism, axial comatic aberration, three-foldastigmatism, third order spherical aberration, star aberration, andfour-fold astigmatism. Also, ω denotes the complex coordinates on theobject plane. Here, tilting the input electron beam along a tilt angle τallows writing χ(ω) as shown next. The tilt angle τ is here expressed incomplex coordinates.

$\begin{matrix}{{\chi \left( {\omega + \tau} \right)} = {{Re}\begin{Bmatrix}{{\frac{1}{2}\omega \; \overset{\_}{\omega}\; {C_{1}(\tau)}} + {\frac{1}{2}{\overset{\_}{\omega}}^{2}{A_{1}(\tau)}} + {\omega^{2}\overset{\_}{\omega}\; {B_{2}(\tau)}} + {\frac{1}{3}{\overset{\_}{\omega}}^{3}{A_{2}(\tau)}} +} \\{{{\frac{1}{4}\omega^{2}{\overset{\_}{\omega}}^{2}{C_{3}(\tau)}} + {\omega^{3}\overset{\_}{\omega}\; {S_{3}(\tau)}} + {\frac{1}{4}{\overset{\_}{\omega}}^{4}{A_{3}(\tau)}} + \ldots}\mspace{14mu}}\end{Bmatrix}}} & {{Formula}\mspace{14mu} 2}\end{matrix}$

In Formula 2, C1(τ), A1(τ), . . . , respectively express the aberrationcoefficients when the electron beam is tilted.

Each aberration coefficient during beam tilt is expressed by the sum ofthe electron beam tilt angle τ, and the aberration coefficient whenthere is no beam tilt. Considering for example aberration coefficientsup to third-order, the defocus (C1(τ)) expressed via the tilt will be asshown below.

C ₁(τ)=Re[C ₁+2C ₃τ τ+4B ₂τ+6S ₃τ]  Formula 3:

Likewise, the dual symmetrical astigmatism (A1(τ)) expressed via thetilt becomes as follows.

A ₁(τ)=A ₁+2A ₂ τ+2 B ₂ τ+C ₃τ²+6 S ₃τ τ+3A ₃ τ ²  Formula 4:

The C1(τ) and A1(τ) contain all aberration coefficients in non-tilt beamoptical system up to third-order as shown in Formula 3 and Formula 4. Ifeven higher order aberration coefficients are included here in Formula1, then χ(ω) can be utilized as a polynomial at any desired order for ω.

Moreover, even when χ(ω) is expressed in polynomial form at a desiredorder, C1(τ) and A1(τ) will of course be expressed in a form containingthe aberration coefficient at the desired order prior to sloping thebeam. In other words, all aberration coefficients for a desired ordercan be found if the function for C1(τ) and A1(τ) and their coefficientare known.

Next, when expressing the direction that the input beam is irradiated ascomplex notation, the tilt angle t for the lens optical axis and theazimuth angle φ on the lens surface can express τ as shown below:

τ=te ^(iφ)

Adjusting by substituting this into Formula 3, finally allows writingthe following formula in the format used by Formula 3 and 4.

Formula 5:

$\sum\limits_{k = 0}^{n}{{m_{k}(t)}^{\; k\; \varphi}}$

Here, m_(k)(t) is a coefficient expressed in a formula made up of eachaberration in non-tilt beam optical system and the linear coupling of t.The m_(k)(t) (Formula 5) can be found by measuring the C1(τ) and A1(τ)for several azimuth angles φ at a particular tilt angle t and performingleast squares fitting.

FIG. 1 shows an example of this least squares fitting for C1(τ). Thensubstituting τ=te^(iφ) into Formula 3 yields the following expression.

C ₁(t,φ)=Re[(C ₁+2C ₃ t ²)+4B ₂ te ^(iφ)+6S ₃ t ² e ^(2iφ])  Formula 6:

When e^(iφ)=cos φ+i sin φ is substituted in here, the Formula 3 finallybecomes the following.

$\begin{matrix}{{C_{1}\left( {t,\varphi} \right)} = {\left( {C_{1} + {2C_{3}t^{2}}} \right) + {4{Re}\; B_{2}t\; \cos \; \varphi} - {4{Im}\; B_{2}t\; \sin \; \varphi} + {6\; {Re}\; S_{3}t^{2}\cos \; 2\varphi} - {6{Im}\; S_{3}t^{2}\sin \; 2\varphi}}} & {{Fomula}\mspace{14mu} 7}\end{matrix}$

Therefore, measuring the value for C1(t, φ) while varying φ at a certainfixed value t, and using the value thus obtained for least squaresfitting in (of Formula 7, allows finding values for (C1+2C3t2), ReB2,ImB2, ReS3, ImS3 from each term of the coefficient. Moreover, the valuefor C3 can be found from the (C1+2C3t2) value by measuring the C1 valuebeforehand while there is no beam tilt (beam is not tilted). Theaberration coefficients in the non-tilt beam optical system can in thisway be found from the values measured for C1(t, ( ) while the beam istilted.

The same calculation can also be made for A1(t, ( ). Here, A1(t, ( )contains terms relating to A2, A3 that do not appear in the C1(t, ( )form, so A2, A3 can be found for the first time by checking A1(t, ( ).All the aberration coefficients in the non-tilt beam optical system canbe found by measuring the A1(t, ( ) and C1(t, ( ) values in this way.

All aberration coefficients in the non-tilt beam optical system can befound by simultaneously solving for the m_(k) (t) that was obtained.

The aberration coefficients calculated in the above steps are then usedto calculate the aberration corrector (aberration compensator)correction quantity and in this way make the aberration correctoradjustment.

Applying the technique as described in these embodiments to scanningcharge particle beam devices that scan an object with a charged particlebeam and acquire image information, allows calculating the aberrationcoefficient with higher accuracy than in the related art. Moreover, theaberration compensator (aberration corrector) adjustment can be madewith higher accuracy than the related art. The technique described inthese embodiments is effective on scanning electron microscopes (SEM),scanning transmission electron microscopes (STEM) and also on focusedion beam (FIB) devices.

First Embodiment

This embodiment describes in detail a scanning electron microscope (SEM)containing a four to eight electrode for an electrical field gravimetricaberration corrector. The aberration corrector described in thisembodiment contains multiple stages of multi-electrode lenses and iscapable of correcting higher-order aberrations. The scanning electronmicroscope of this embodiment is characterized in using a standardspecimen formed with steps to acquire the SEM image and then utilizingthe SEM image from that standard specimen to calculate the aberrationcoefficient.

FIG. 2 shows a block diagram concept view of the system structure of theSEM of the present embodiment. The SEM shown in FIG. 2 includes aspecimen chamber 22 containing a specimen stage 30 for holding thespecimen 32; an electron optical column 21 to irradiate an electron beamonto the specimen 32 and, including a function to detect the emittedsecondary electrons or reflected electrons and output a signalcontaining the detected results; a control power supply 24 forregulating the electrical current and voltage supplied to each componentof the electron optical column; an data processing device 23 forprocessing the signals that were output and performing different typesof processing; and an image display 12 for displaying image data thatwas processed by the data processing device 23. The interior of theelectron optical column 21 and the specimen chamber 22 are maintained ata high vacuum during device operation. Electrons emitted from anelectron gun 1 pass through a first condenser lens 2 and a deflectioncoil 3 and are input to an aberration corrector 4. In this embodiment,the aberration correction system 20 is made up of a first condenser lens2 of aberration corrector 4, the deflection coil 3, and the secondcondenser lens 5.

This aberration correction system 20 includes the first condenser lens 2to guide the electron beam along a track parallel to the aberrationcorrector 4, the deflection coil 3 that functions to tilt the input beamused for aberration measurement relative to the optical axis, theaberration corrector 4 to actually correct the aberration (aberration),and the second condenser lens 5 for forming a crossover of electronsemitted from aberration corrector 4 at a suitable position above theobjective lens 7.

The operation of the multiple electrodes that together make up theaberration corrector 4 and the track of the electron beam passing insidethese multiple electrodes are described next. The aberration corrector 4is made up of a four-stage multi-electrode electron lens mounted alongthe optical axis. A static four-electrode field and an eight-electrodefield are overlapped in the first and the fourth stages of themulti-electrode lens. A static four-electrode field and magneticfour-electrode field are overlapped in the second and third stages. Theelectron beam track passing along the optical axis can in this way beindependently changed in the X and Y directions by adjusting each stageof these electrical and magnetic fields. Aberrations (aberrations) inthe electron beam passing through the interior of the aberrationcorrector can be corrected in an operation where: the first stageelectrostatic multi-electrode field isolates the input electron beamalong an X track and a Y track and, the second and third stagesseparately eliminate aberrations along the X track and Y track (moreaccurately, the structural elements of electron optical column 21 suchas the objective lens 7 generate an inverse aberration), and the fourthstage returns the isolated tracks to the original state.

The SEM of this embodiment includes a mechanism that tilts the beaminput onto the object point of the objective lens, relative to theoptical axis of the objective lens. The SEM of this embodiment forexample contains a second stage deflector 3 in the upper part of theaberration corrector. This second stage deflector 3 forms a beam whosecenter axis is a certain tilt angle and azimuth angle relative to theoptical axis of objective lens 7. The memory 9 stores informationregarding the beam azimuth angle and tilt angle. This information ischecked at times such as when acquiring image data and calculating theaberration coefficients.

The second condenser lens 5 passes along and shrinks the electron beamthat passed through the aberration corrector 4. A scan coil 6 scans theelectron beam that passed through the second condenser lens 5 andirradiates the electron beam via the objective lens 7 onto a specimen 32mounted on a specimen stage 30. Secondary charged particles such assecondary electrons and reflected electrons emitted from the specimen 32are detected by a secondary electron detector 8 as secondary chargedparticle signals, output from the electron optical column 21 asbrightness distribution type image data to the data processing device23, and finally are stored in memory 9.

A computing unit 10 inside the data processing device 23 uses thereceived image data to calculate the aberration coefficients for eachaberration/aberration remaining on the optical system. After calculatingthe aberration coefficients, the amount of electrical current needed bythe corrector to handle this aberration is calculated, and thecorrection signal is then calculated by finding the difference versusthe electrical current or electrical voltage values currently beingapplied to the multi-electrode lens of aberration corrector 4. Thiscorrection signal is forwarded to the control power supply 24 by way ofan output unit 11, and is consequently fed back to the SEM side ofaberration corrector 4. The image display 12 outputs the specimen imagedata and information on the calculated aberration coefficient.

The SEM of this embodiment utilizes a standard specimen on which stepsare formed as shown in FIG. 3 in order to acquire images in one batchfor setting the defocus data and aberration quantity. The standardspecimen shown in FIG. 3 is a structure with a flat plane of lengths Lx1through Lx5, and a width Ly; on a step-shaped specimen stand in stepsLz1 through Lz5. This flat plane forms the beam irradiation surfacewhere the beam is irradiated, and on which spherical specimens such ashole patterns and metal vapor particles are mounted. Information on thelengths Lx1-Lx5, width Ly and further the steps Lz1-Lz5 of the beamirradiation surface is stored in the memory 9. This information isreferred to during different types of image processing and whencalculating the aberration coefficients.

The sequence used in the SEM of this embodiment when correcting theaberrations is described next while referring to FIG. 4 and FIG. 7. FIG.4 shows a flow chart of the entire process of this embodiment.

As part of the setup, the stage is shifted and the standard specimenutilized for the focus alignment and aberration adjustment is scannedand the usual optical axis alignment (STEP1), focus alignment andastigma adjustment (STEP2) are performed. During the focus alignment andastigma adjustment in STEP2, the vicinity of center of a stair for thestandard specimen shown in FIG. 3 is set as the referential stair, andthe equipment is adjusted in a state where the astigma had beencorrected and the focus was aligned relative to the specimen on thereferential stair. The control power supply 24 executes these focusalignments and astigma adjustments based on step information Lz storedin the memory 9, and specimen height information from the specimenheight measurement unit not shown in the drawing in FIG. 2. The standardspecimen images are next acquired in this state (STEP3). When acquiringthe reference image, the scanning range of the electron beam is set tothe length Lx and width Ly, images of the standard specimen surfaceirradiated by the beam are acquired together and, an image data (pixeldistribution data) on a surface area equivalent to the beam irradiationregion on each step among the batch of acquired images is removed byusing the length information Lx1-Lx5 and width information Ly on eachstep. This removed image data is linked to reference information thatmatches the number of steps on the beam irradiation surface, and storedin the memory 9. The operation for STEP3 is now complete.

One example of the specimen image (specimen image in a state where focusalignment of the referential stair of the standard specimen is complete)acquired in STEP3 is shown in FIG. 5. To simplify the description,reference numerals 1-5 showing the stairs are assigned to each stair onthe specimen. When the focus has been aligned with the referential stair(stair No. 3), the focus is now aligned (just focus state) with thereferential stair on the specimen but is in an under-focus state versusthe specimen (stairs Nos. 4 and 5) on the side above the referentialstair, and in the same way is in over-focus state versus the specimen(stairs Nos. 1 and 2) on the side below the referential stair. Thedeflection coil 3 is operated in this state to form a tilt beam and, theC1(τ) and A1(τ) are measured. The operation up until now has been thesetup for calculating the aberration, and the aberration coefficientsare actually calculated in STEP4.

FIG. 6 is a flow chart showing in more detail the aberration coefficientmeasurement flow for the flow chart of FIG. 4. The deflection coil 3 isadjusted from the above described focused state to input an electronbeam having a tilt angle τ and an azimuth angle φ versus the objectpoint (STEPS 11, 12). The range of the tilt angle τ and azimuth angle φ,and step size of the change in azimuth angle and tilt angle are presetand stored within the memory 9. The data processing device 23 reads outthe setting information for τ, φ stored in the memory and transfers thisinformation to the control power supply 24. The control power supply 24controls the deflection coil 3 based on the data processing device 23instructions and irradiates the electron beam onto the specimen at thespecified tilt angle and azimuth angle. Images of the irradiated surfaceon the standard specimen are acquired all together in this state(STEP13), and image data matching the beam irradiation region on eachstep is extracted from the acquired batch of images (STEP14). The justfocus position and the in-focus position at the beam tilt angle τ arenext estimated by image processing the image data for beam irradiationregions on each stair (STEP15). When estimating the just focus position,the length in the X direction and the Y direction for each pattern edgeare calculated, and the position of the stair number with the smallesttotal length sum in the X direction on each pattern is set as the Xdirection just focus state position, and the position of the stairnumber with the smallest total length sum in the Y direction on eachpattern is set as the Y direction just focus state position. Whenestimating the in-focus position for example, the step number positionon each pattern edge with the smallest difference in lengths in the Ydirection and X direction is estimated as the in-focus position. Alltypes of image processing in the above description are implemented bythe computing unit 10 in FIG. 2. Besides the above description, varioustypes of algorithms may also be used to decide the just focus state andthe in-focus state.

Next, the C1(τ) and A1(τ) are measured using the position information(i.e. information expressed by step numbers) that was obtained and thebeam irradiation surface length information Lx1-Lx5 as well as the stepinformation Lz1-Lz5 stored in the memory 9 (STEP16).

The method for executing the processing steps in STEP14-STEP16 isdescribed next in detail using FIG. 7. The image processing andarithmetic processing are implemented by the computing unit 10 usingsoftware stored in the memory 9 the same as the flow in FIG. 4.

FIG. 7 is a drawing showing the change in the specimen image before andafter sloping (tilting) the beam, and the interrelation between thedefocus and aberration. After beam sloping, the specimen image focusstate is changed from that prior to sloping the beam. Moreover thesloping applies an aberration to the beam so that the in-focus position(where the two aberrations are at positions balanced 90 degrees awayfrom each other) is different from the pre-sloping position. In theexample shown in FIG. 7, the in-focus position after beam sloping hasshifted one step higher from step number 3 to 4 compared to before thebeam sloping. Moreover, the aberration appears at the step number 1-3positions in the Y direction after beam sloping, and the aberrationappears in the X direction at the step number 5 position. In FIG. 7, thedefocus data is the difference between the height of the step number 4which is the in-focus state, and the height of step number 3 which isthe just focus state (state where beam is tilted) because the defocusdata is defined by the difference between the in-focus state positionand the just focus state position. The defocus data C1(τ) when the tiltangle is τ can therefore be found from each piece of positioninformation on step numbers 3 and 4 and information (step informationLz3) on the difference in height at each position.

The astigmatic difference is defined as the difference in focal pointdistance in two intersecting directions along the optical axis of thecharged particle beam. Therefore the astigmatic difference is regardedas the difference in distances between the step in the X direction wherethe focus is best aligned, A1(τ) and the step in the Y direction wherethe focus is best aligned. In the case shown in FIG. 7, the step number3 position is the step where the focus is best aligned in the Xdirection, and the step number 5 position is the step where the focus isbest aligned in the Y direction so the aberration quantity A1(τ) isfound as the difference between the heights of step numbers 3 and 5.

Aberrations in the 45 degree direction are set in the same way. Morespecifically, the line profile may be taken along the 0 degree directionand 45 degree direction of both the reference image and the capturedimage, and changes in the focus along the line then measured. Here the 0degrees and 45 degrees define the proper rectangular coordinate systemon the acquired image. The 0 and 45 degrees may be set as the referencealong the X axis. In the processing for steps 13-16 described above,C1(τ) and A1(τ) are measured as one step measurement, and the data forφ, C1(τ) and A1(τ) stored in the memory 9.

When STEP16 ends at 0≦φ<2π, a decision is made on whether C1(τ), A1(τ)were measured at all azimuth angles (STEP17). In this embodiment, therange of φ is set as the range from the initial phase. If the azimuthangle is 2π at the point in time where step 16 ends, then the step 18loop terminates and the process shifts to step 19. If the azimuth angledid not reach 2n then the process returns to step 12, the deflectioncoil 3 is adjusted, and the azimuth angle φ is increased just by thetrack width δφwhile maintaining the tilt angle (. The process in steps13-16 is then repeated for the newly set azimuth angle (+((and the C1((), A1(( ) are measured (STEPS 13-16).

When finished measuring C1(( ), A1(( ) for the range of (and (set permk(t), the coefficient of Formula 5 is calculated by least squaresfitting (STEP18), the value thus obtained is used to solve for eachaberration coefficient (STEP19). All desired aberration coefficients innon-tilt beam optical system can be found in this way.

After using the above process to measure the aberration coefficientscontained in the current optical system, the size of the correctionsignal conveying the aberration coefficients that were obtained to theaberration corrector is set (STEP5), the correction signal is fed backto the aberration corrector, and the aberration is corrected by changingthe strength of the aberration corrector field according to thecorrection signal (STEP6). The post-correction reference image is thencaptured (STEP7), and the aberration coefficient after correction ismeasured by using the same technique as shown in FIG. 7 (STEP8). Adecision is next made on whether to re-correct according to the actualcorrection amount obtained via the post-correction coefficients, or toend the process (STEP9).

As already described, utilizing the stair-shaped specimen stand of thisembodiment allows measuring the defocus data and aberration quantityfrom one image, and calculating the aberration coefficient at high speedwith few images. The example for this embodiment described using anaberration corrector with four to eight electrodes needless to sayhowever other multi-electrode lens structures may be used in thisaberration corrector.

Second Embodiment

The charged particle beam apparatus of this embodiment is a SEM(scanning electron microscope) with the object of setting the defocusdata and the astigmatic difference quantity at high speed in thestair-shaped standard specimen by mounting a piezoactuator in thespecimen stage.

FIG. 8 shows the structure of the device of the second embodiment. Adescription of sections with the same operation and function as in FIG.2 is omitted. In FIG. 8, a stage 31 driven in the direction of thedevice height and utilizing a piezoactuator is mounted in the specimenstage 30. The range of changes in focus due to aberration will change asthe aberration correction progresses so that when using a stair-shapedspecimen stand, multiple specimen stands of different heights must beprepared so that no focus mismatches will occur regardless of the levelon the stair-shaped specimen stand during the aberration correction.However, the stair-shaped specimen stand can be adjusted so that thestand center is always at the in-focus state by changing the height ofthe stair-shaped specimen stand driven along the device height and usinga piezoactuator. Out-of-focus states and aberration quantities can inthis way be measured more accurately than when using the step-shapedspecimen stand alone.

Third Embodiment

The third embodiment shows the case where this invention is applied to alength-measurement SEM.

FIG. 9 is a block diagram showing the system structure of thelength-measurement SEM of this embodiment. The length-measurement SEM isa device that measures the length between two points on measured imagedata by making pixel calculations. The length-measurement SEM of thisembodiment contains a specimen pre-setup chamber 23 for guiding thespecimen into the device, the SEM shown in FIG. 2 contains a specimenchamber including a specimen stage 30 for holding the specimen 32, anelectron optical column 21 for irradiating an electron beam onto thespecimen 32, and includes a function for detecting the emitted secondaryelectrons or reflected electrons and outputting the signal detectionresults, a electron optical system control device 38 to regulate theelectron optical column, an data processing device 22 to process thesignal that was output and perform different types of arithmeticprocessing, and an image display 12 to display image data processed bythe data processing device 22. However the functions and operation ofeach component are approximately the same as described in the firstembodiment so redundant descriptions are omitted.

A gate valve 25 partitions the specimen chamber in the device main unitand the specimen pre-setup chamber 23. To feed the specimen into thedevice, the gate valve opens, and a specimen conveyor mechanism 39 feedsthe specimen into the specimen chamber on the main device. The devicemakes adjustments using the standard specimen 33 mounted on the specimenstage 30.

The length-measurement SEM of this embodiment contains a boostingelectrode 34 above the magnetic field objective lens 5. Applying anelectrical field to this boosting electrode forms an electrostatic lens.Fine adjustments can be made to the focus by changing the strength(intensity) of this electrostatic lens. The voltage applied to theboosting electrode 34 is varied by the controlling the boosting powersupply 35 and in this way vary the focus (boosting focus). A voltage(retarding voltage) for forming a decelerating electrical field isapplied by the retarding power supply 36 to the input electron beam inthe specimen stage 30. However regulating this retarding voltage allowsadjusting the focus (retarding focus). The response to excitationcurrent in the magnetic field objective lens is delayed due to magneticaftereffects so that high speed focus changes can be performed byadjusting the retarding voltage and boosting voltage rather than usingthe objective lens excitation current.

In contrast to the first and second embodiments where one image is madeto hold multiple focus information by applying changes the specimenheight, in this embodiment the focus position is changed in the electronbeam itself to measure the defocus data so no stepped-shaped specimensuch as in FIG. 3 is needed.

The following method may for example be utilized to find C1(τ), A1(τ)from the captured image.

First of all, each focus point on the specimen image is captured whilevarying the electron beam focus by changing the boosting voltage or theretarding voltage.

Next the directional degree of sharpness (directional differential inthe sum of squares for 0°, 45°, 90°, 135°) from each captured image iscalculated. This directional degree of sharpness forms a function forfocus values in each direction.

Setting the respective focus values where the directional degree ofsharpness reaches a peak as p0, p45, p90, p135, allows expressing thesize δ and the direction a of aberration A1(τ) and C1(τ) by thefollowing formula.

$\begin{matrix}{\delta^{2} = {\left( {{p\; 0} - {p\; 90}} \right)^{2} + \left( {{p\; 45} - {p\; 135}} \right)^{2}}} & {{Formula}\mspace{14mu} 8} \\{\alpha = {\frac{1}{2}\; {\tan^{- 1}\left( \frac{{p\; 45} - {p\; 135}}{{p\; 0} - {p\; 90}} \right)}}} & {{Formula}\mspace{14mu} 9} \\{{C\; 1(\tau)} = \frac{{p\; 0} + {p\; 45} + {p\; 90} + {p\; 135}}{4}} & {{Formula}\mspace{14mu} 10}\end{matrix}$

In other words, the defocus data is equivalent to the average of themaximum values for the directional differential of squares in the fourdirections. The astigmatic difference amount can be found by using atrigonometric function to fit the focus position where the differentialof the sum of squares in all astigmatic directions is a maximum, andthen calculating the difference between the maximum and minimum valuesobtained from the fitting curve (when the aberration difference isdefined by the difference between the minimum and maximum values).Utilizing this method allows simultaneously finding the defocus data andaberration difference in one operation.

The length-measurement SEM of this embodiment can set values for thetilt angle τ and range of azimuth angle φ and its step size ((by usingthe GUI interface displayed on the image display device 12. If adjustingto correct the aberration manually, then the user can set the beam tiltand check results on this screen. The tilt angle r is for example set byinputting the specified value in the tilt angle setting unit 50. The((can be set by designating the number of images to capture per onecycle of tilt beam images on the azimuth angle setter unit 51. Moreoverthe type of aberration/aberration to be corrected can be specified byselecting the type of aberration displayed on the correction specifierunit 52. The computing unit 10 generates the form for C1(τ), A1(τ) shownin Formula 5, in a format containing the specified type of aberrationcoefficient, and uses it during the least squares fitting shown in step19 of FIG. 6. This method calculates the desired aberration coefficientselected by the device user. The aberration coefficient calculated inthis way appears on the result display unit 52. A correction processselector 54 sets the process for starting correction and endingcorrection. The correction results can be confirmed since the imagedisplay unit 55 shows specimen images from both before and aftercorrecting the aberration/aberration.

Utilizing the electrostatic lens focusing of this embodiment allowsmoving the focus at high speed, measuring the defocus data andaberration quantity in a short time, and finding the aberrationcoefficients at high speed.

1. A charged particle beam apparatus comprising: a charged particleoptical system to scan a primary charged particle beam on a specimen,detect secondary charged particles emitted from the specimen due to thescanning and output the detection results as a secondary chargedparticle signal; a control unit for controlling the charged particleoptical system; a data processing device to process the secondarycharged particle signal that was output and acquire two-dimensionaldistribution information on pixels matching the area scanned by theprimary charged particle beam, wherein the charged particle opticalsystem includes: an aberration corrector to reduce the aberrationgenerated by the charged particle optical system; and a unit to guidethe charged particle beam from the optical axis of the charged particlebeam in a tilted state onto the specimen, and wherein the dataprocessing device: calculates an out-of-focus amount and astigmaticdifference in the charged particle optical system from multipletwo-dimensional distribution information of a directional degree ofsharpness acquired by scanning the specimen with the primary chargedparticle beam in a tilted state and also while changing the degree offocus, and calculates an optical order aberration coefficient to use inthe charged particle optical system, by utilizing the two-dimensionaldistribution information obtained from scanning the specimen in a statewhere the primary charged particle beam is not tilted, and thecalculated out-of-focus amount and astigmatic difference.
 2. The chargedparticle beam apparatus according to claim 1, wherein the dataprocessing device calculates the out-of-focus amount and astigmaticdifference based on the respective focus values that the directionaldegree of sharpness reaches a peak.
 3. The charged particle beamapparatus according to claim 1, wherein the charged particle opticalsystem contains a magnetic field objective lens, and wherein the controlunit for the charged particle optical system changes the degree of focusby adjusting the magnetic field objective lens.
 4. The charged particlebeam apparatus according to claim 1, wherein the data processing device:calculates the out-of-focus amount, astigmatic difference and astigmaticdirection in the charged particle optical system from multipletwo-dimensional distribution information of a directional degree ofsharpness acquired by scanning the specimen with the primary chargedparticle beam in a tilted state and also while changing the degree offocus, and calculates an optical order aberration coefficient to use inthe charged particle optical system, by utilizing the two-dimensionaldistribution information obtained from scanning the specimen in a statewhere the primary charged particle beam is not tilted, and thecalculated the out-of-focus amount, astigmatic difference and astigmaticdirection.
 5. A charged particle beam apparatus comprising: a chargedparticle optical system to scan a primary charged particle beam on aspecimen, detect secondary charged particles emitted from the specimendue to the scanning and output the detection results as a secondarycharged particle signal; a control unit for controlling the chargedparticle optical system; a data processing device to process thesecondary charged particle signal that was output and acquiretwo-dimensional distribution information on pixels matching the areascanned by the primary charged particle beam, wherein the chargedparticle optical system includes: magnetic field objective lens; anaberration corrector to reduce the aberration generated by the chargedparticle optical system; and a unit to guide the charged particle beamfrom the optical axis of the charged particle beam in a tilted stateonto the specimen, and wherein the data processing device: calculates anout-of-focus amount and astigmatic difference in the charged particleoptical system from multiple two-dimensional distribution informationacquired by scanning the specimen with the primary charged particle beamin a tilted state and also while changing a degree of focus, andcalculates an optical order aberration coefficient to use in the chargedparticle optical system, by utilizing the two-dimensional distributioninformation obtained from scanning the specimen in a state where theprimary charged particle beam is not tilted, and the calculatedout-of-focus amount and astigmatic difference, wherein the control unitfor the charged particle optical system changes the degree of focus byadjusting the magnetic field objective lens.
 6. The charged particlebeam apparatus according to claim 5, wherein the data processing device:calculates the out-of-focus amount, astigmatic difference and astigmaticdirection in the charged particle optical system from multipletwo-dimensional distribution information of a directional degree ofsharpness acquired by scanning the specimen with the primary chargedparticle beam in a tilted state and also while changing the degree offocus, and calculates an optical order aberration coefficient to use inthe charged particle optical system, by utilizing the two-dimensionaldistribution information obtained from scanning the specimen in a statewhere the primary charged particle beam is not tilted, and thecalculated the out-of-focus amount, astigmatic difference and astigmaticdirection.
 7. A charged particle beam apparatus comprising: a chargedparticle optical system to scan a charged particle beam on a specimen,detect a transmission electron transmitted from the specimen due to thescanning and output the detection results as a transmission electronsignal; a control unit for controlling the charged particle opticalsystem; a data processing device to process the transmission electronsignal that was output and acquire two-dimensional distributioninformation on pixels matching the area scanned by the charged particlebeam, wherein the charged particle optical system includes: anaberration corrected to reduce the aberration generated by the chargedparticle optical system; and a unit to guide the charged particle beamfrom the optical axis of the charged particle beam in a tilted stateonto the specimen, and wherein the data processing device: calculates anout-of-focus amount and astigmatic difference in the charged particleoptical system from multiple two-dimensional distribution informationacquired by scanning the specimen with the charged particle beam in atilted state and also while changing a degree of focus, and calculatesan optical order aberration coefficient to use in the charged particleoptical system, by utilizing the two-dimensional distributioninformation obtained from scanning the specimen in a state where thecharged particle beam is not tilted, and the calculated out-of-focusamount and astigmatic difference.
 8. The charged particle beam apparatusaccording to claim 7, wherein the data processing device calculates thecorrection quantity of the aberration corrector by utilizing thecalculated aberration coefficient.
 9. The charged particle beamapparatus according to claim 7, wherein the charged particle opticalsystem contains a magnetic gravimetric field objective lens including anelectrostatic lens, and wherein the control unit for the chargedparticle optical system changes the degree of focus by adjusting theapplicable electrostatic lens.
 10. The charged particle beam apparatusaccording to claim 9, comprising a storage unit to store informationrelating to the range width of the focus adjustment.
 11. The chargedparticle beam apparatus according to claim 7, wherein the dataprocessing device calculates the out-of-focus amount and dualsymmetrical aberration from the two-dimensional distributioninformation, and calculates the aberration coefficient by using theapplicable calculated out-of-focus amount and dual symmetricalaberration.
 12. The charged particle beam apparatus according to claim7, wherein the charged particle optical system contains a magnetic fieldobjective lens including, and wherein the control unit for the chargedparticle optical system changes the degree of focus by adjusting themagnetic field objective lens.
 13. The charged particle beam apparatusaccording to claim 7, wherein the data processing device: calculates theout-of-focus amount, astigmatic difference and astigmatic direction inthe charged particle optical system from multiple two-dimensionaldistribution information of a directional degree of sharpness acquiredby scanning the specimen with the charged particle beam in a tiltedstate and also while changing the degree of focus, and calculates anoptical order aberration coefficient to use in the charged particleoptical system, by utilizing the two-dimensional distributioninformation obtained from scanning the specimen in a state where thecharged particle beam is not tilted, and the calculated the out-of-focusamount, astigmatic difference and astigmatic direction.